1. Field of the Invention
The invention relates to a method for driving an electroluminescent lamp to emit light at a brightness set level, in which a variable bias voltage is provided to said electroluminescent lamp, wherein the variable bias voltage is increased as a function of a duration of on-time of the electroluminescent lamp.
2. Description of the Related Art
An electroluminescent lamp that is used as a light source for backlighting displays or panels such as, for example, in automotive applications is a thin structure formed of a layer of electroluminescent material sandwiched between two conductive layers. When a sufficiently high bias voltage is applied across the two electrodes, a resulting electric field causes the intermediate electroluminescent layer to emit light that is visible through the transparent electrode. In general, the brightness of light emitted increases with the bias voltage, up to a maximum allowable voltage level. Apart from the change in brightness of the light emitted due to the change in bias voltage, electroluminescent lamps suffer from brightness decay which progresses in time due to aging. The aging process giving rise to the brightness decay occurs only during the actual on-time of the electroluminescent lamp in question, i.e., during the periods in which the electroluminescent lamp is actually emitting light. Accordingly, the half-life of the lamp must be distinguished from the life time of the lamp, wherein the half-life is defined as the actual on-time from initial turn-on until the lamp dims to half the original brightness at constant nominal bias setting and life time is defined as the actual on-time, elapsing from initial turn-on until the lamp dims to its lowest acceptable brightness level while adequately increasing the bias voltage up to its maximum allowable upper limit value. The lowest acceptable brightness level may be assumed to correspond to, e.g., 50% of the original brightness level. The original brightness level is hereinafter also referred to as brightness set level.
An example of a display driver circuit implementing an on-time dependent increase of the variable bias voltage of an electroluminescent lamp to emit light at a brightness set level being stabilized against aging as such is known from U.S. Pat. No. 5,493,183. This known display driver circuit provides open loop compensation of the typical on-time dependent, non-linear brightness decay of electroluminescent lamps by using an inverter controlled by a microprocessor. The microprocessor controls the inverter in accordance with a table containing data which is the inverse function of the non-linear brightness decay. The microprocessor tracks the total on-time of the electroluminescent lamp in question and selects the appropriate data from the table for driving the lamp. However, due to manufacturing tolerances, the non-linear brightness decay of electroluminescent lamps varies from one lamp to the other. The open loop control is fixed to the inverse function of the non-linear brightness decay stored and does not provide for any correction in mismatch between the stored data on the one hand and the actual brightness decay of the electroluminescent lamp used on the other hand. Due to this control mismatch the lamp may not be properly used to its full lifetime.